The present invention relates to laser destruction of human calculi.
Lithotripsy, the crushing of human stones into easily removable fragments, is an old enterprise. Frere Jacques (of the French Folk song) was an itinerant lithotomist 300 years ago.
Modern medical practitioners, until recently, viewed gall bladder, kidney and ureteral stones as being removeable only by open surgery. Bladder stones, however, have been treated by endoscopic lithotripsy for many years. Physicians view the interior of the bladder with a cystoscope. This device is passed via the urethra into the bladder. An elongated lens combined with an eyepiece is passed along and attached to a sheath so that an image of the interior of the bladder may be seen. Typically, a cystoscope contains a means for passing irrigating fluid into and out of the bladder, a means for removing tissue samples from the bladder and a means of visualization.
Various methods of crushing bladder stones have been combined with the cystoscope: for example, mechanical crushing, mechanical drilling and more recently, ultrasonic disintegration.
The first ultrasonic lithotripsy of bladder stones occurred in Germany in 1968. The combined means of a cystoscope, a piezo-electric ultrasonic transducer and an interchangable ultrasonic probe was used. The probe is passed into the bladder under visual control. It will disintegrate some bladder stones on contact. Ultrasonic lithotriptors will not break hard stones. Each probe may be used for approximately 100 pulses before it must be replaced. Ultrasonic lithotriptors are consequently limited in use and expensive to operate.
The interior of the kidney (known as the pelvis of the kidney) may be visualized with the nephroscope, which is similar in use and appearance to the cystoscope. This endoscope is said to be used percutaneously where the sheath is passed into the kidney through the abdominal wall. Since the nephroscope became available, ultrasonic lithotripsy of kidney stones has become common practice. This practice is described, for example, by J. E. Lingeman in his article "Current concepts on the relative efficacy of percutaneous nephrostolithotomy and extracorporeal shockwave lithotripsy" in World Journal of Urology 5, 229, 1987, and by G. E. Brannen, W. H. Bush, R. E. Correa, R. P. Gibbons, and J. S. Elder in their article "Kidney Stone Removal, percutaneous versus surgical lithotomy" in Journal of Urology, 133, 6, 1985.
Approximately 50% of symptomatic urinary stone disease results from stones in the ureter. Ureteroscopes which may be either rigid or flexible are typically longer and thinner than other endoscopes. Ultrasonic lithotripsy is used very little in the ureter because of the rigid and relatively large diameter of the ultrasonic probe. See for example the article by J. L. Huffman, D. H. Bagley, H. W. Schoednberg and E. S. Lyon, "Transurethral removal of large ureteral and pelvic calculi using ureteroscopic ultrasonic lithotripsy" in the Journal of Urology, 130, 31, 1983.
Endoscopic destruction of gall stones has not become common medical practice although it is clearly possible.
The introduction in the early 1980's of the so called extracorporeal shockwave lithotriptor (E.S.W.L.) resulted in a significant improvement in the treatment of kidney stones. In this type of device a shock wave is created by an underwater high current spark discharge. By means of an elliptical acoustic reflector this shock wave is focused through the soft abdominal wall and kidney onto the kidney stone. The shock wave has little damaging effect on soft tissue but fractures hard stones. The resulting crushed stone is passed through the urinary tract. The E.S.W.L. is described, for example, by C. Chaussy, E. Schmiedt, D. Jocham, W. Brendel, B. Forsmann and V. Waltham in their article "First clinical experience with extracorporeally induced destruction of kidney stones by shock waves" in the Journal of Urology 127, 417, 1982 and more recently by G. W. Drach in "Report of the United States Cooperative Study of Extracoporeal Shock Wave Lithotripsy", in the Journal of Urology, 135, 1127, 1986.
The E.S.W.L. is less effective for the treatment of ureteral stones. This is partly because of the difficulties in avoiding the pelvis and partly because ureteral stones are not immersed in liquid so that shock wave coupling to the stone is less efficient. For a discussion of this limitation of E.S.W.L. see the article by J. Graff, J. Pastor, P. Mach, W. Michel, P. J. Funhe, and T. Senge, "Extra corporeal shock wave Treatment of Ureteral Stones" in the Journal of Urology, 137, 143A, 1987.
The laser has recently been used as an alternative means for the destruction of ureteral stones. An extensive review is provided by the book entitled "Laser Lithotripsy" edited by R. Steiner, Springer Verlag, 1988. Typically the laser is focused into a fibre optic which is passed along a rigid or flexible ureteroscope. The ureteroscope is inserted through the bladder into the ureter and the stone is visualized. The operator may in this way direct intense pulses of light onto or in the vicinity of the stone and thus cause fracturing and disintegration of the stone. In the current state of the art as described by Steiner in "Laser Lithotripsy", two laser sources are effective in lithotripsy: the pulsed dye laser and the Q-switched NdYAG laser.
For a description of the operation of the pulsed dye laser, see the article by G. M. Watson, S. Murray, S. P. Dretley and J. A. Parrish in the Journal of Urology, 138, 195, 1987. Typically, pulses of visible light in the spectral region from 445 to 577 nm and of duration of 1 .mu.s to 300 .mu.s are used. The energy of each pulse is typically 10 mJ to 60 mJ. Typically the distal laser fibre tip touches the surface of a human stone which is immersed in water. A light pulse of sufficient intensity is then passed along the fibre. When a threshold of energy is exceeded a brilliant flash of light is emitted by the stone. This flash is accompanied by the disintegration of part of the stone. The white flash is accompanied by a clicking sound. The light emission by the stone and the shock which fractures the stone are attributed to the formation of a laser induced plasma. A discussion of this mechanism is in the paper by G. M. Watson.
Although the dye laser is reported to fragment all human calculi but the very hard uric acid and calcium oxalate monohydrate stones, it has several clinical shortcomings. The fragments of stone produced by dye laser lithotripsy tend to be relatively large compared with the powder produced by externally applied shock waves. Extraction of debris following lithotripsy is consequently a problem. If sharp debris is impacted in the ureter, for example, this represents a significant clinical problem.
Another difficulty encountered by users of dye laser lithotriptor is the variability of effectiveness. Formation of a plasma in a laser irradiated stone results from absorption of light by pigments in the stone. Pigmentation is variable. In renal stones it varies from almost white to dark brown. The laser conditions needed for formation of a plasma are difficulty to predict because the color of the stone is not known prior to lithotripsy. The size of fragments created by the dye laser is typically larger than fragments produced by a shock wave. The size of dye laser-produced fragments increases as the laser energy increases. Fragment size also increases as the depth of light penetration into the stone increases. Extraction of large fragments of stone from a ureter following dye laser lithotripsy is a significant clinical problem.
In an attempt to find a laser lithotriptor which does not depend upon the photochemical properties of human concrements, several groups have developed Q-switched NdYAG laser lithotriptors. See, for example, the article by F. Wondrazek and F. Frank, "Devices for intracorporal Laser induced Shock Wave Lithotripsy" in Laser Lithotripsy, Springer Verlag, 1988. Typically, in this type of device a 10 ns laser pulse from a Q-switched NdYAG laser with the wavelength of 1.06 .mu.m is transmitted through a fibre from the laser to the vicinity of a stone. The stone is immersed in irrigating liquid which is typically water or saline and essentially transparent to the 1.06 .mu.m laser light. Consequently, light emerging from the fibre will pass freely through the liquid and strike the stone or tissue in the vicinity of the stone and be absorbed. In the method of Q-switch NdYAG laser lithotripsy a small lens is attached to the output or distal end of the fibre so that light emerging from the fibre is focused a few millimeters from the said lens. If the intensity of laser light is sufficiently high a small plasma will form at the focus in a manner which is known as laser breakdown. The plasma absorbs light emerging from the fibre so that little light strikes the stone or tissue. The absorption results in a rapid deposition of energy into a small volume of plasma. The expansion of the hot plasma gases is limited by the surrounding liquid and an explosive increase in pressure results. The resulting shock wave radiates from the plasma in all directions at the speed of sound in the liquid. In the method of Q-switched laser lithotripsy a small parabolic reflecting surface is attached to the distal end of the fibre so as to reflect part of this shock wave toward a stone placed typically 3 mm from the plasma. Repeated impact of these shock waves on a stone tends to disintegrate the stone so that a lithotripic effect results. This method has the advantage that the photochemical composition of the stone does not influence the lithotripsy and the resulting stone fragments are similar to those produced by E.S.W.L. The disadvantages of this method are: the lens holding chamber is typically larger than two millimeters in diameter so that access to stones through a ureteroscope is limited; the larger rigid ureteroscope must be used; and the exposure of the lens to the plasma shock wave will destroy the lens after approximately 100 pulses. The limitations of this type of lithotripsy are discussed by S. Thomas, J. Pensel, W. Meyer and F. Wondrazeh in "The Development of an Endoscopically Applicable Optomechanical Coupler for Laser Induced Shock Wave Lithotripsy" in Laser Lithotripsy, Ed. R. Steiner, Springer Verlag, 1988.
In the present invention a method of laser lithotripsy uses laser light which is absorbed by water. Pulses of light absorbed by water in the vicinity of the stone cause erosion of the stone.
The geometry of the present laser lithotriptor includes a pulsed Neodymium Yttrium Aluminum Garnet, (NdYAG) laser operating at the wavelength 1.44 .mu.m. This laser is described in the U.S. patent application Ser. No. 06/933,10 filed by John Tulip in 1986. Unlike other emissions from the NdYAG laser, for example at the wavelengths of 1.06 .mu.m and 1.32 .mu.m, NdYAG laser emission at 1.44 .mu.m is stongly absorbed by water. The depth over which this laser light is essentially absorbed by water is approximately 0.3 mm. Absorption of laser energy by water results in water heating and even vaporization if the laser light is sufficiently intense. In one example, this effect is used to vaporize human body tissue water with a focussed NdYAG 1.44 .mu.m laser beam for surgical purposes. If the NdYAG laser is flashed or pulsed, the radiation may appear in intense, short bursts or pulses of energy. Such pulses of 1.44 .mu.m radiation quickly heat and vaporize water.
Another part of this laser lithotriptor system is a quartz fibreoptic. Pulses of light from the said 1.44 .mu.m NdYAG laser may be focussed into and along quartz fibreoptic. In particular quartz fibreoptic with very low OH content, commonly known as anhydrous quartz fibreoptic, effectively transmits 1.44 .mu.m light. This fibreoptic may be used to direct laser pulses to the vicinity of body stones by passing the fibreoptic along such endoscopes as the ureteroscope.
Another part of this laser lithotriptor geometry is water in the vicinity and in contact with body stone. It is common practice to irrigate the bladder, ureter or kidney with water or water solutions such as saline during endoscopic examination. Natural body fluids are also composed largely of water. Therefore body stones are normally immersed in water.
We have discovered that a combination of these three means, applied correctly, will rapidly erode even the hardest calcium oxalate body stones.
It has been discovered that a microscopic explosion may occur at the distal, or output, tip of the fibreoptic when the fibre tip of the fibreoptic is placed in water and a pulse of 1.44 mm radiation is transmitted through the fibre. Apparently no plasma occurs in this explosion, no visible emission can be seen and no lens is placed at the fibre tip. The laser pulse period is typically up to tens of milliseconds which is much longer than the 10 nanosecond pulse period of the Q-switched 1.06 mm NdYAG laser. The velocity of sound in water is sufficiently high that a shockwave created by a millisecond long laser absorption induced micro explosion would travel away from the fibre tip in a time which is much less than the laser pulse period. Unlike the Q-switched NdYAG 1.06 .mu.m laser, the 1.44 .mu.m laser is absorbed by water. Absorption by water causes a micro-explosion.
1.06 mm radiation is absorbed very weakly by water so that plasma formation at the fibre tip is necessary to cause a micro explosion. With 1.44 .mu.m radiation rapid heating and vaporization of water in the vicinity of the fibre tip causes a rapidly expanding vapour bubble. The expansion of this bubble is inhibited by the fibre tip and the surrounding water so that a miniature laser absorption-induced explosion occurs. This explosion is accompanied by a sharp audible click. These laser absorption induced micro explosions have been examined using high speed shadow photography which reveal the shape and velocity of the expansion front of the explosion. These images show a sharp, well-defined front which is similar in appearance to the domed explosion front commonly photographed during nuclear explosions.
In one example, a distal fibre tip was cleaved flat and perpendicular to the fibre axis. Shadow photography did not show a spherically shaped expansion as might be expected, but a highly directional expansion away from the tip in the direction of the fibre axis. The expansion velocity was initially typically 2 millimeters per millisecond and resulted in a bubble typically 5 mm in length and 2 mm in diameter.
In another example a fibre was drawn to a point by heating the fibre with a flame and pulling two parts of the fibre apart. This pointed fibre tip produced very different shadow photographs. A spherical expansion occurred close to the tip of the fibre point and resulted in an approximately spherical bubble of about two to three millimeters diameter.
Continued pulsing of a fibre tip for many thousands of pulses does not apparently influence the formation of absorption induced micro explosions in water. Unlike the Q-switched 1.06 .mu.m NdYAG laser lithotriptor, damage to the fibre tip surface (which occurs with use and which appears similar to a sand blasting effect) does not influence the creation of micro-explosions in water since the formation of micro-explosions using 1.44 .mu.m radiation does not rely upon the use of a focusing lens.
It has been discovered that micro-explosions, in water at the tip of a fibreoptic and created by pulsed 1.44 .mu.m radiation, will erode human stones. If a human stone is immersed in water and the said laser activated fibre tip is brought in contact or near the stone, rapid erosion of the stone surface will occur and fine, powdery, debris will appear in the water. This debris rapidly obscures the stone from view. In contrast, if the stone is placed in air and the said laser activated fibre it is brought near the stone, melting of of the stone surface occurs and no erosion is observed. The presence of water around the stone is a necessary means for laser erosion to occur.
If the 1.44 .mu.m laser-activated fibre tip touches the surface of the stone when high average laser power is applied, a bright flash (associated with plasma formation) may occur. Melting of the fibre tip, melting of the stone and adhesion of the fibre tip to the stone frequently accompany flashing. This undesirable plasma formation is unlike the dye laser lithotriptor (which requires the fibre to touch the stone and cause a flash in order to fragment the stone.) Despite flashing and melting by touching the stone with the fibre tip, it is possible to push the fibre several centimeters through even very hard stone. This is achieved by repeatedly removing the fibre from the small cylindrical hole created by pushing the fiber optic into the stone in order to permit water to enter the hole.
Experiments have been undertaken to find the best distance between the stone surface and the activated fibre tip. In one, fibre was passed through a channel in a cystoscope so that the tip was visible through the lens of the cystoscope--a stone and the fibre tip could be observed simultaneously under water in a rigid container. Small pieces of chalk, (a widely-accepted substitute for soft human calculus,) of known size were fixed by epoxy to the base of the water container. A small wire was attached to the cystoscope to control the distance between the fibre and the chalk. The time required to totally erode a chalk sample was measured as the distance to the chalk was varied. The fibre tip was moved manually across the chalk in a circular pattern which resulted in circular grooves which progressively eroded the chalk. The time required to destroy the chalk sample was used as a measure of erosion rate. Both the pointed and flat fibre tips eroded the chalk most effectively with their tips within two millimeters of the sample. Little improvement in erosion rate resulted from placing the fibre tip closer than 2 millimeters. When the laser was used at low average laser power the best erosion rate occurred when the fibre tip was in contact or close to contact with the stone. The pointed fibre tip generally produced a faster erosion rate than the flat fibre tip. Examination of the interaction of the micro-explosions and a stone was performed with shadow photography. The explosion expansion front from a flat fibre tip strikes the stone surface and tends to be deflected laterally and perpendicular to the direction of the fibre axis. The explosion expansion front from a pointed fibre strikes the stones and tends to be deflected away from the stone along the axis of the fibre. For this reason a greater momentum change of the expansion occurs for a pointed fibre tip which may explain the greater stone erosion rate which results from use of a pointed fibre tip.
Light at 1.44 .mu.m is absorbed by water within 0.5 mm. Q-switch 1.06 .mu.m wavelength light from the NdYAG laser and 0.53 .mu.m wavelength light from the dye laser pass through water essentially unabsorbed. If the direction of the fibreoptic tip is deflected so that light does not strike the stone, the light can pass freely through the surrounding water and strike tissue. In order to avoid accidental injury, the average power from both of these laser lithotriptors is limited to a few watts. This results in slow stone destruction rates for both the Q-switch NdYAG laser lithotriptor and the dye laser lithotriptor.
In order to assess the safety of 1.44 .mu.m laser lithotripsy, animal experiments were performed. Exposed skin on an anesthetized small animal flank was submerged in water and exposed to the activated fibre tip. In the example of a flat fibre tip, tissue injury was observed only when the axis of the fibre was perpendicular to the skin surface. When the fibre was parallel and in contact with the skin, no injury was observable for an average laser power of 45 watts emerging from the fibre. When the fibre was perpendicular to the skin, injury was observed when the fibre tip was less than 3 mm from the skin with an average laser power of 45 watts. In the example of pointed fibre tip, injury was observed only when the fibre tip was within 1 mm of the skin with an average power of 45 watts. Consequently, for the pointed fibre tip it is necessary to essentially touch tissue to cause accidental injury.